The invention relates to a speed governor mechanism for use in a pneumatically powered tool and more specifically to a governor mechanism which senses the rate of change of speed as well as the speed of the motor in the pneumatic tool and controls air flow to the motor of the tool in response to these parameters.
Many prior art tools incorporating rotary air powered vane motors also incorporate means to control the maximum delivered torque by terminating the flow of air to the motor as it slows and begins to stall due to the tightening of a fastener or the completion of a similar production step. One such mechanism comprehends the use of a back pressure sensor which automatically shuts off the flow of air to the air motor as the back pressure and torque resistance increase. Tools which incorporate such a pressure sensing system are disclosed in U.S. Pat. Nos. 3,373,824 and 3,608,647. While satisfactory in many applications, this pressure sensitive mechanism has a disadvantage in that it is sensitive to line pressure variations. That is, a pressure surge in the line can prematurely stop the tool, thereby improperly performing the production step. A more serious problem, however, occurs when line pressure is too low to activate the shutoff. This condition can easily result from excessive air consumption, inadequate air supply or partially blocked air lines. As a result, the tool simply fails to shut off. Consequently, the production step may, again, be improperly performed; but of greater concern is the safety of the operator. If the tool is of the high torque output variety, the operator may be subjected to an unexpected and dangerous stall jerk from the tool.
Other prior art tools have used mechanical means such as clutches or ratchet-type devices to control the torque delivered to a fastener or a joint. Such devices are often complex and with regard to the ratchet-type devices are especially subject to wear and concommitant loss of calibration. Devices of this type also waste energy inasmuch as they generally do not remove power from the driving motor but rather merely disconnect the motor from the driving element. Furthermore, it should be noted that these devices will produce a stall jerk if a pressure drop of sufficient magnitude to reduce motor torque below the clutch setting is encountered.
A third class of shutoff control devices encompasses speed control mechanisms. These mechanisms are basically centrifugal governor assemblies which terminate the flow of air to the air motor when the R.P.M. drops below a fixed value. Since these stall control mechanisms sense the slowing of the motor associated with the increase in torque due to the tightening of a fastener or the completion of a production step, they provide more accurate control of the torque output of the tool and minimize improperly completed production steps. Since they are generally insensitive to variations in air line pressure and their mechanism are relatively free from wear, they also retain their accuracy and repeatability. Such devices are disclosed in U.S. Pat. Nos. 3,791,458 and 3,904,305.
These patented shutoff mechanism represent substantial improvements over previous designs. However, torque limiting and shutoff performance of these tools in one particular application has been found to somewhat unsatisfactory. This particular application comprehends those manufacturing steps wherein torque requirements may remain at a very low level throughout most of the operating cycle but increase substantially and rapidly to a higher value as the step is completed. Such a torque-time characteristic is associated with the fastening of two or more relatively unyieldable members by a threaded fastener. During the rundown of the fastener, the torque requirement is minimal and the motor operates near its unloaded speed. As the fastener grounds against the members, the torque requirement increases rapidly to a maximum while the drive motor slows rapidly and stops. Such an application is in contradistinction to the fastening of two structures, one or both of which are yieldable, and which slowly compress and exhibit a gradually incrasing torque requirement and a time-torque parameter. In applications where the torque requirement rises rapidly, it has been found that fasteners are both erraticly tightened and often overtightened. Operator fatigue is also greatly increased due to the generally increased stall jerk which is transmitted thereto.
One cause of such unsatisfactory performance is the momentum of the rotating drive components of the pneumatic tool. That is, although the centrifugal speed governor may terminate, the air flow to the air motor and speed reducing components will be transferred to and absorbed by the fastener and it will generally be overtorqued. Stated in another manner, the torque increase and stall occurs so rapidly that the speed control governor is incapable of terminating the air flow to the motor either as rapidly or as uniformly as is required to uniformly tighten a fastener or complete a manufacturing step.
A second cause of such unsatisfactory performance is the result of the vane type air motor commonly used as a source of power in such tools. At low r.p.m., such motors tend to pulse as the vanes rotate past inlet and exhaust ports. A pulse at a crucial moment of the final torguing of a fastener may also result in an improperly completed manufacturing step.